Electronic devices capable of providing a pulse output signal for therapeutic purposes are now well known, and such devices are known, for example, to include transcutaneous electrical nerve stimulating (TENS) devices and muscle stimulating devices. It is also well known that such devices commonly include externally accessible controls for effecting parameter changes, including changes in pulse width and intensity of the delivered pulses (see, for example, U.S. Pat. No. 4,014,347).
Conventional TENS and muscle stimulator devices commonly use constant current feedback to regulate the charge per pulse. The feedback signal, representing the peak current, is compared with the reference voltage to establish the height of the rectangular pulses, and the reference voltage is usually derived from a variable control device, such as a potentiometer, which serves to control the stimulation intensity. In other words, stimulation amplitude is commonly controlled by varying the peak current of the rectangular current pulses.
It has been found that, at least with respect to TENS devices and muscle stimulator devices, that large pulse widths are useful for short term stimulation since they are more comfortable to the user than narrow pulse width for the stimulation of large muscles. Conversely, narrow pulse widths are more comfortable to the user for stimulating small muscle groups, and narrow pulse widths are particularly well suited for use for long term stimulation which often occurs, for example, in effective stimulation by a TENS unit.
It is usually necessary, or at least desirable, that the pulse width be tuned (i.e., varied) while using the unit. However, since the charge per pulse varies directly with the pulse width, unless the peak current is also adjusted whenever the pulse width is changed, the charge per pulse will also change.
Since stimulation intensities perceived by a user are approximately proportional to the delivered charge per pulse, a wide pulse will acquire a large amount of charge when the intensity control is moved only slightly in the intensity increase direction. Conversely, when the pulse width is narrow, changing the peak current a small amount, by moving the intensity control in the intensity increase direction, will produce a small amount of increase in the charge per pulse. Since pulse width affects the sensitivity of the intensity control where both are utilized in a device, using a pulse width, for example, of greater than about 200 microseconds, causes conventional amplitude controls to become overly sensitive. As a result, pulse widths greater than about 200 microseconds are difficult to control with an ordinary potentiometer-type amplitude, or intensity, control.
In addition, the pulse width of the pulses being delivered sometimes causes confusion with respect to the amount of charge being delivered, or at least perceived to be delivered. For example, if the pulse is 100 ma peak and 200 microseconds wide, the pulse contains 20 microcoulombs charge. While such a charge would normally be too strong to be tolerated with small electrodes such as are commonly used with TENS units, for example, such a charge could easily and safely be tolerated with comfort if sufficiently large electrodes are utilized (i.e., the area of application is made sufficiently greater).
It was therefore found that if the intensity was increased as the pulse width was decreased, and if the intensity was decreased as the pulse width was increased, the charge delivered by the pulses can be controlled to produce a more constant level of stimulation. A strength-duration curve to effect this end was suggested in connection with a diagnostic apparatus (see, U.S. Pat. No. 2,808,826), and has also been suggested for use in connection with TENS units (see U.S. Pat. Nos. 4,431,002 and 4,453,548). In addition, such a curve has been suggested for use in connection with mechanical coupling of pulse width and amplitude controls in a stimulator (see U.S. Pat. No. 4,340,063), such a curve has been suggested for use in connection with scanning output parameters through predetermined ranges to periodically achieve optimum stimulating currents (see U.S. Pat. No. 4,210,151), and such a curve has also been suggested for use in conjunction with strength-duration-rate modulation in a TENS device (see U.S. Pat. No. 4,759,368).
It has also been heretofore suggested that the intensity of the delivered output pulses can be controlled, and gradual increase in such intensity is utilized, for example, in the NuWave.RTM. TENS unit now being sold by the assignee of this invention.
Thus, while pulse compensation has heretofore been suggested in connection with therapeutic-type devices, improvements and/or additions to such devices could further improve such devices.